1. Field of the Invention
This invention relates generally to encoding information, and relates more particularly to a system and method for encoding read-only information on storage media.
2. Description of the Background Art
Efficient, economic, and reliable storage of digital data is an important consideration of manufacturers, designers, and users of computing systems. In magneto-optical storage devices, digital data is typically stored in tracks located on rotating disks of magneto-optical (MO) storage media. Close positioning of the adjacent disk tracks maximizes the amount of stored data on a storage disk, thus providing significant economic benefits to system manufacturers and users. Therefore, system designers frequently seek new and improved methods of reducing track pitch to permit greater storage capacity on the storage media. As track pitch is reduced, differentiating between the tracks becomes of even greater importance for efficient and reliable storage of data.
Referring now to FIG. 1, a plan view of a front surface 112 of a magneto-optical storage medium 110 is shown. In MO storage devices, digital data is typically written into and read from a series of concentric or spiral tracks 114 located within a plurality of data sectors 177 on surface 112 of storage medium 110. The digital data is read from and written to surface 112 of storage medium 110 by projecting a laser-generated light spot from a magneto-optical head onto a selected track 114 while storage medium 110 is rotating, and then sensing the polarization of light reflected back from storage medium 110.
The head must be accurately positioned above track 114 of rotating storage medium 110 during a read/write operation on that track. Many factors, for example imperfections in track symmetry, may cause the head to be positioned slightly off the center of track 114. Positional correction of the head is therefore required for acceptable performance during a read/write operation.
One prior art position correction technique utilizes a diffraction pattern to generate a position error signal from grooves that are positioned between tracks on the storage medium. Another correction technique utilizes a plurality of servo sectors 178. Each servo sector 178 contains read-only information that indicates the position of the head on storage medium 110. This read-only information may be in the form of position marks permanently embossed on surface 112 of storage medium 110 at manufacture. The position marks may be used to generate a position signal, which may then provide feedback to compensate for position errors by adjusting the position of the head.
Referring now to FIG. 2(a), a diagram of position marks on sample storage media tracks within a servo sector 178 is shown. FIG. 2(a) includes sample tracks 0 through 4. Five tracks are presented for purposes of illustration, however storage medium 110 typically contains a significantly greater number of tracks. Furthermore, FIG. 2(a) depicts track 0 through track 4 as straight, whereas in practice they are typically circular.
As shown in FIG. 2(a), each track has three associated position marks which may be repeated at selected intervals along their corresponding track. The position marks are formed by depressions in surface 112 of storage medium 110. The ellipses shown in FIG. 2(a) represent the full-width-half-maximum dimensions of the depressions. The full-width-half-maximum dimensions of a depression are its dimensions at a plane located halfway between surface 112 and the bottom of the depression. When optical spot 220 (the full-width-half-maximum dimensions of the light spot from the head) travels over a position mark, the diffraction pattern is such that most of the light is not reflected back to the head. A resulting pulse occurs in a detected reflectivity signal that is based on the amount of light reflected back from storage medium 110 to the head.
Referring now to FIG. 2(b), a drawing of a reflectivity waveform corresponding to position marks 210, 212, and 214 is shown. During a read/write operation on track 4, the head is positioned over track 4 as storage medium 110 rotates at a selected rate of speed. The head initially encounters position mark 210, which is a radial bar created by overlapping elliptical depressions. When optical spot 220 passes over position mark 210, the amplitude of reflected light is reduced, generating negative-going sync pulse 230 at time 240. Ideally, a position mark would cause the reflectivity signal to fall to zero as optical spot 220 passes directly over the mark. In practice, position mark 210 is detected when the reflectivity signal becomes small, as represented by sync pulse 230.
Next, the head encounters position mark 212, which is positioned at a specified perpendicular distance off-center from track 4. Position mark 212 generates a negative-going pulse xe2x80x9cAxe2x80x9d at time 242. The amplitude of pulse A is relatively less than the amplitude of sync pulse 230 because optical spot 220 does not pass directly over position mark 212. Next, the head encounters position mark 214, which is positioned at the same specified distance off-center of track 4, but in the opposite direction of position mark 212. Position mark 214 generates a negative-going pulse xe2x80x9cBxe2x80x9d at time 244. The amplitude of pulse B is also relatively less than the amplitude of sync pulse 230. A radial position error signal (PES) for the head may then be obtained by taking the difference of the peak reflectivity amplitudes of pulse A and pulse B.
In some designs for MO storage devices, optical spot 220 has a linear plane of polarization with a direction that cannot be controlled. The amount of light reflected back to the head as the head passes over a position mark may be affected by the unpredictable direction of the plane of polarization of optical spot 220.
For example, optical spot 220 may have a plane of polarization that is parallel to position mark 210. A diffraction pattern created by optical spot 220 passing over position mark 210 may be such that a significant amount of light is reflected back to the head. A resulting pulse in the reflectivity signal may not be large enough to indicate the presence of a position mark.
Undetected position marks create errors in the read-only information being read from servo sectors 178. Errors in the read-only information read from servo sectors 178 cause the MO storage device to perform unreliably. Therefore, an improved system and method are needed to encode read-only information on storage media.
In accordance with the present invention, a system and method are disclosed to encode read-only information on storage media. One embodiment of the present invention is implemented in the context of a magneto-optical storage device. In the magneto-optical storage device, read-only information is encoded in servo sectors on surfaces of magneto-optical storage media.
One embodiment of the present invention includes a magneto-optical storage medium, a plurality of position marks disposed on a surface of the storage medium, and a light beam directed towards the position marks to produce a reflection of the light beam from the storage medium. The light beam is preferably a single-frequency laser beam.
The position marks are configured whereby the reflection of the laser beam is not responsive to a plane of polarization of the laser beam. In one embodiment, the position marks comprise substantially circular pits. The reflection from each of the substantially circular pits is not affected by the direction of the plane of polarization. The dimensions of the substantially circular pits depend on a wavelength of the light beam and a numerical aperture of a lens that directs the light beam towards the position marks. Each of the substantially circular pits has a depth of approximately one-quarter of the wavelength of the light beam.
The substantially circular pits may be disposed on the surface of the storage medium in rows. The rows are configured such that a distance between centers of the substantially circular pits is equal to about. one-half the wavelength of the light beam divided by the numerical aperture of the lens. The reflection of the light beam as the beam moves along a row is approximately constant, even though the substantially circular pits do not overlap. Therefore, the present invention more efficiently and effectively encodes read-only information on storage media.